The present invention relates to a fixing device of a printer, and more particularly to a fixing device for an electrophotography basis printer. Further, the invention relates to a fixing device for drying a solvent contained in an ink jet printer.
There are known a fixing device which uses a halogen lamp for its heating source, and a fixing device which uses an electromagnetic induction heater for a heat source. In the related fixing device using a halogen lamp for its heating source, as shown in FIG. 29, a halogen lamp 23 cylindrical in shape is located within a fixing roller 21 hollowed, while being centered. The halogen lamp 23, when fed with current, emits an electromagnetic wave containing mainly infrared rays, and it reaches the inside of the fixing roller 21. It is converted into heat in the roller. The heat is transferred to the outside of the fixing roller 21. A recording sheet 25 bearing a marking material (toner image 24) is heated and fixed in a state that it is nipped between the fixing roller 21 and a pressure roller 22 under pressure of a spring 27. A temperature sensor 28 senses a temperature on the outside of the fixing roller 21. A temperature controller 29 controls current fed to the halogen lamp 23 in accordance with the sensed temperature, whereby a temperature of the fixing roller 21 is stabilized.
In the fixing roller 21, a core bar is shaped like a pipe, and made usually of a member of iron or aluminum family. The outer surface of the core bar is covered with a release layer for preventing an offset of the marking material. If necessary, the outer surface of the pipe-like core bar is additionally covered with an elastic layer (of a material, e.g., silicone rubber). The fixing roller, when covered with the elastic layer, is capable of pressing a toner image against the irregular surface of the recording sheet 25 under an appropriate pressure. Accordingly, toner is uniformly fused, so that the resultant picture is good in quality. A heat conductivity of the elastic layer is usually lower than that of the core bar metal. Therefore, much time is taken till a temperature on the surface of the fixing roller 21 reaches a predetermined level, and the thermal response is not good. The fixing roller 21 is supported at both ends on a structure frame by means of bearings.
The core bar of the pressure roller 22 is made of iron, stainless aluminum or the like, and its circumferential outer surface is coated with an elastic layer. In the case of the both-side printing, a release layer is applied also to the pressure roller 22. The spring 27 applies a nip load to the fixing roller 21 and the pressure roller 22, whereby those rollers form a nip.
For examples of the related printer fixing device of the type in which the electromagnetic induction heater is used as a heat source, a fixing device in which a heating coil is located within the fixing roller is disclosed in Japanese Patent Publication Nos. 8-179647A and 9-160413A.
Further, there is disclosed, in Japanese Patent Publication No. 8-129315A, a fixing device in which a core of a closed magnetic path is located passing through the fixing roller and a heating coil is located outside the fixing roller.
Still further, there is disclosed, in Japanese Patent Publication No. 8-63022A, a fixing device in which a plurality of coils are wound, and one coil develops a magnetic field whose direction is opposite to that of a magnetic field developed by its adjacent coil.
In the fixing device disclosed in Japanese Patent Publication No. 8-179647A, a magnetic flux developed by the coil feeds from the coil center and along the fixing roller. Therefore, a magnetic path formed is the sum of a length of the fixing roller and a length of the coil, and hence it is elongated. Accordingly, its reluctance is large and its magnetic density is reduced. An eddy current generated is proportional to a square of the magnetic density. Accordingly, the fixing device suffers from such a problem that an efficiency of the induction heating is reduced.
In the fixing device disclosed in Japanese Patent Publication No. 9-160413A, a bobbin and a coil wound on the bobbin are located at the central portion of the hollowed fixing roller. A magnetic flux developed by the coil feeds from the coil center and along the fixing roller. The structure of it resembles the one mentioned above, and suffers from a similar problem.
In the fixing device disclosed in Japanese Patent Publication No. 8-129315A, the fixing roller and the core form a magnetic path with a magnetic gap. The magnetic flux feeds along the fixing roller. The magnetic path formed is the sum of a length of the fixing roller and a length of the core, and hence is elongated.
In the fixing device disclosed in Japanese Patent Publication No. 8-63022A, the construction is applicable only for such a case where the coils merely adjacent to each other are arranged so as to develop magnetic fields whose directions are opposite to each other. Additionally, it is impossible to quantitatively grasp its connection difference.
Another fixing device is proposed in Japanese Patent Publication No. 8-16005A, in which a plurality of cores and coils are formed in a fixing film, and those coils are selectively used in accordance with a size of a recording sheet. However, a high frequency drive circuit and a controller are not clearly disclosed. To flow an AC current into a plurality of coils, it is necessary to select a capacitor of a resonance inverter in accordance with the inductance and resistance values of the coil. This necessitates the provision of the resonance inverter for each coil, and hence results in increase of the cost to manufacture.
Accordingly, the first object of the present invention is to increase the heating efficiency, to efficiently heat the fixing device in accordance with the width of a recording sheet, and to enhance a stability of the temperature control of a fixing roller portion which the recording sheet passes.
The following examples may be further enumerated for the related fixing device which uses the induction heater as the heat source.
There is disclosed, in Japanese Patent Publication No. 8-129313A, a fixing device in which an elastic layer and a metal sleeve are placed within a rotary heater.
There is disclosed, in Japanese Patent Publication No. 9-50199A, a fixing device which is formed with a fixing film, a coil, and a pressure roller including a core bar, an elastic layer, a heating layer, and a release layer.
Further, as is shown in FIG. 30, there is known a fixing device in which a fixing roller 31 is constructed with a metal fixing sleeve disposed around the circumferential outer surface of a holder within which a coil 33 held on a core 34.
In the fixing device disclosed in Japanese Patent Publication No. 8-129313A, a coil being wound on a U-shaped core is used for heating the metal sleeve of 10 to 15 μm. An AC magnetic field is present only near the core. Accordingly, the magnetic coupling of the coil with the metal sleeve is weak, so that the heating efficiency is small. Additionally, a portion of the metal sleeve in which the eddy current is generated by the AC magnetic field is limited to a portion of the sleeve except a portion thereof that the magnetic field enters at a right angle. Where the core is used, a region from which the magnetic field emanates is limited, and hence the heating by the eddy current is limited in its amount. This results in ineffective heating of the metal sleeve.
In a case where the heating source is a halogen lamp, only the portion of it that is irradiated with the electromagnetic wave from the halogen lamp is heated to be high in temperature, and a local rise of a surface temperature of the fixing roller tends to occur. As a result, the release layer is easy to deteriorate.
In the fixing device disclosed in Japanese Patent Publication No. 9-50199A, the fixing film and the heating layer of the pressure roller are inductively heated by the AC magnetic field developed from the coil. Of the magnetic field developed from the coil, only the magnetic field leaking from the fixing film contributes to generate the eddy current. Accordingly, the reduction of the magnetic coupling of the fixing film with the coil is the precondition in constructing the fixing device. The way of heating it from the fixing film greatly contributes to the heating of the toner since it directly contacts with the toner. However, the way to heat the pressure roller less contributes to the heating of the toner.
In the fixing device shown in FIG. 30, the pressure roller is driven to rotate, and then the flexible fixing sleeve and the holder of the coil are rotated with a frictional contact with the former. Therefore, a rotation speed difference is like to occur between the recording sheet. Accordingly, the fixed image is easy to be disarranged. Particularly in the case of high speed color printing, three color toners have been transferred to the recording sheet, slip is easy to occur between the fixing sleeve and the recording sheet. Since the coil is located within the fixing sleeve, the heat generated in the fixing sleeve and the heat generated spontaneously in the coil are confined within the fixing sleeve. Accordingly in the high speed printing, temperature rise is great.
In the related fixing device using a halogen lamp as a heating source, the fixing roller is hollowed, and it is placed therein. Accordingly, the fixing roller should be a hard roller, and the pressure roller should be a soft roller. Accordingly, the nip formed is a forward nip as shown in FIG. 31A, which causes the recording sheet to bend toward the fixing roller. In this case, the fused toner tends to stick to the fixing roller. Accordingly, the recording sheet is likely to wind itself around the fixing roller. In other words, the releasing property is poor.
Conversely, when the fixing roller is located on the upper side while the pressure roller is located on the lower side, the reverse nip is formed between them as shown in FIG. 31B. After passing through this nip, the recording sheet tends to bend downward. In this case, a force by bending the recording sheet is greater than a force at which the fused toner sticks to the pressure roller surface. Accordingly, the releasing property is good. When the nip is a horizontal nip, which is medium between the forward nip and the reverse nip as shown in FIG. 10C, the release property is good as in the case of the reverse nip.
In the case of a recording sheet being folded to be double layered, such as an envelope, the creases of the recording sheet which are formed when passing through those nips will be different. In this case, the recording sheet is double layered. Accordingly, when a curvature of the nip is large, the recording sheet is easy to be creased. To avoid the formation of the crease, it is desirable that the curvature of the nip is gentle. To secure good repeatability of the recording sheet and to prevent the crease formation of the recording sheet, it is preferable to use the horizontal nip.
Accordingly, the second object of the present invention is to reduce a rise time of heating in the fixing roller, and to enable the formation of a horizontal nip.
The following examples may be further enumerated for the related fixing device which uses the induction heater as the heat source.
There is disclosed, in Japanese Patent Publication No. 8-137306A, a fixing device in which a conductive fixing belt is used, and an electromagnetic induction coil is attached to the reverse side of the belt.
There is disclosed, in Japanese Patent Publication No. 8-286534A, a fixing device in a fixing roller is constructed with a heating member, an exciting coil and a fixing film, and the exciting coil for inductively heating the heating member is incorporated thereinto.
The fixing device disclosed in Japanese Patent Publication No. 8-137306A uses a fixing film of a small thermal capacity in order to reduce a rise time of the heating. Further, it uses two rollers, a drive roller and a tension roller, for driving it to rotate. Accordingly, the thermal capacity of those two rollers is essentially present.
The related fixing device disclosed in Japanese Patent Publication No. 8-286534A, a drive force output from the pressure roller frictionally drives a recording sheet and the fixing film to rotate. Accordingly, when the frictional force varies, a moving speed of the recording sheet and a rotational speed of the fixing film becomes instable. As a result, a frictional force present between the rotating fixing film and the fixed nip forming member also affects them. In particular at high speed rotation, the frictional force is likely to be instable.
Accordingly, the third object of the present invention is to provide a fixing device which is capable of lessening a curl of the recording sheet to surely form a substantially horizontal nip.
The following examples may be further enumerated for the related fixing device which uses the induction heater as the heat source.
There is disclosed, in Japanese Patent Publication No. 10-161445A, a fixing device in which a temperature sensor is provided on a metal thin plate, whereby a temperature sensing response and a sensing accuracy are improved.
There is disclosed, in Japanese Patent Publication No. 10-91019A, a fixing device in which a thermistor is provided near a nip portion of a fixing film, and a shielding member is provided between the thermistor and a magnetic field generator, whereby erroneous detection of temperature and erroneous operation are prevented.
In the fixing device disclosed in Japanese Patent Publication No. 10-161445A, since the temperature sensor is provided on the metal plate, a temperature sensed varies depending on a degree of the heating of the metal plate by the AC magnetic field. An object to be sensed is a temperature of the fixing roller heated by the AC magnetic field or a temperature of it coming in contact with a toner image having transferred onto a recording sheet. It is required that a temperature of the temperature sensor is closest to a temperature of the sensed object. In this connection, it is difficult to heat the metal plate and the fixing roller in the same degree by the AC magnetic field, and to radiate the equal quantities of heat from them so that they are put at the equal temperature.
In the fixing device disclosed in Japanese Patent Publication No. 10-91019A, the shielding member is made preferably of a material being conductive and having a high permeability. Such a material has also a nature that it is easily heated by the AC magnetic field. In other words, the shielding member is heated by the AC magnetic field to increase its temperature. This will lead to a temperature difference between the temperature sensor and the sensed object.
Accordingly, the fourth object of the present invention is to improve accuracy of temperature sensing.
The following examples may be further enumerated for the related fixing device which uses the induction heater as the heat source.
There is disclosed, in Japanese Patent Publication No. 7-295414A, a fixing device in which a plurality of coil segments each spirally coiled are separately arranged in the axial direction of the fixing roller, and the coil segments located on the outer side are dense in coil density, while the coils located on the inner side are coarse in coil density, in order to secure a uniform temperature distribution of the fixing roller.
There is disclosed, in Japanese Patent Publication No. 8-179647A, a fixing device in which a coil is located in a fixing roller and arranged such that both ends of the coil are located close to the fixing roller, and the central portion of the coil is located apart from the fixing roller. With such an arrangement, the eddy currents generated at both ends of the fixing roller are different from the eddy current generated in the central portion, thereby securing a uniform temperature distribution in the fixing roller.
There is disclosed, in Japanese Patent Publication No. 9-306652A, a fixing device in which a temperature distribution is equalized in the fixing roller in a manner that the fixing roller is shaped to be thick at both ends, to increase electrical resistance values there and hence the amounts of heat there.
In the fixing device disclosed in Japanese Patent Publication No. 7-295414A, however, a plurality of sheets each containing the coil segment are laminated together into a bulk, and the laminated ones are electrically interconnected, in order to array the coil segments. Accordingly, it leads the manufacturing cost up.
In the fixing device disclosed in Japanese Patent Publication No. 8-179647A, a magnetic flux flows into the heating layer of the fixing roller in the axial direction. Because of this, a flux density little varies depending on a distance between the coil and the fixing roller. This fact implies that the fixing device is less effectual in compensating for the temperature lowering at both ends.
In the fixing device disclosed in Japanese Patent Publication No. 9-306652A, an AC magnetic field of 20 to 200 kHz is applied to the heating layer. Accordingly, the AC magnetic field developed from the coil penetrates into the heating layer to such a shallow depth corresponding to a depth two to three times as long as a conductor skin thickness, since the conductor skin effect acts. Accordingly, even if a thickness of the heating layer of the fixing roller is varied, a temperature distribution is less equalized in the fixing roller.
Further, in any of the related fixing devices described above, when an iron plate of a housing or the like is located close thereto, it will be heated unless some measure is taken for preventing the AC magnetic field of the coil from leaking outside the fixing device.
As a cause of causing the non-uniform temperature distribution in the fixing roller when the fixing roller is heated by the halogen lamp or the induction heating, the heat radiated from the heat source is lost as heat loss in the form of the heat that effectively heats the fixing roller, and further the heat transferred to the bearings and the structure members and the heat diffused from the surface of the fixing roller by convection. In particular the quantity of the heat transferred to the bearings and the structure members is large. As a result of the heat loss by the heat transfer, the temperature drop is observed at both ends of the fixing roller in the profile of a temperature distribution of the fixing roller surface.
A measure having been taken for this is that a distribution of the heat from the heat source is profiled such that the heat magnitude is large at both ends of the fixing roller, thereby compensating for the heat loss by the heat transfer. Since the heat loss by the heat transfer forms the cause of making the heat distribution non-uniform, to take a measure to impede the heat transfer as well as the measure of profiling the heat distribution of the heat from the heat source is taken is a more effective way to equalize the temperature distribution in the fixing roller. However, there is not such an approach, so far as we know.
Accordingly, the fifth object of the present invention is to reduce the heat loss by the heat transfer in the fixing roller, and hence to lessen the non-uniformity of the magnetomotive force distribution in the axial direction of the fixing roller, and consequently to remove the non-uniformity of the temperature distribution in the fixing roller.
A magnetic field generator for the electromagnetic induction heating, unlike other types of heating members, does not generate heat by itself, and hence its temperature remains low as compared with that of the fixing roller to be heated. Water contained in a sheet of paper as the recording medium is evaporated when the sheet passes through the fixing device since it is heated. Therefore, the downstream end of the fixing device as viewed in the sheet transporting direction is placed in an atmosphere heavy with moisture. In this state, there is a probability that dew condensation occurs on the magnetic field generator.
Accordingly, the sixth object of the present invention is to provide a fixing device capable of reducing a probability of condensing the atmosphere into water drops on the magnetic field generator, and hence improves the durability and safety of the device.